Information recording medium and recording reproducing method thereof

ABSTRACT

An information recording medium for reproducing information by irradiation with a laser beam condensed by an objective lens with a numerical aperture NA includes a disk-shaped substrate and a recording layer disposed on the substrate. On the surface of the substrate, a plurality of prepit regions ( 110 ) and a plurality of data regions ( 120 ) are disposed alternately along spiral or concentric virtual track centers. Each prepit region includes a pair of wobble pits ( 113 ) for tracking servo. and a length L (μm) of the wobble pit along the virtual track center, a wavelength λ (μm) of the laser beam, and a numerical aperture NA satisfy a relationship: 0.3≦L.NA/λ≦0.65

TECHNICAL FIELD

[0001] The present invention relates to an information recording mediumfor reproducing information by irradiation with a laser beam, and arecording/reproducing method thereof

BACKGROUND ART

[0002] Recently, there is a demand for high-density optical disks, andthe track pitch thereof is being narrowed while the linear density isbeing increased. In order to achieve a narrow track pitch, it isrequired to reduce interference with adjacent tracks such ascross-write. Therefore, it is important to conduct tracking control of abeam spot for recording/reproducing with good precision.

[0003] As a method for controlling a beam spot, a push-pull trackingsystem is frequently used. However, this system has problems involved ina shift of an optical axis and a tilt of a disk.

[0004] As a method for controlling a beam spot while reducing an errorof tracking control even when fluctuations such as a shift of an opticalaxis occur, a sample servo tracking system is known. According to thissystem, tracking control is conducted based on a reproduction signalfrom prepit regions disposed separately on a disk. FIG. 17Aschematically shows a conventional configuration of a prepit region onan optical disk adopting the sample servo tracking system.

[0005] Referring to FIG. 17A, in a prepit region, each clock pit 2 isdisposed on a virtual track center 1. Furthermore, a pair of wobble pits3 are disposed at positions shifted by a ¼ track from the virtual trackcenter 1. A pair of wobble pits 3 are composed of a first wobble pit 3 aand a second wobble pit 3 b disposed on different sides of the virtualtrack center 1. An address pit 4 is formed at a predetermined distancefrom the second wobble pit 3 b along the virtual track center 1.

[0006] According to the sample servo tracking system, a tracking erroris detected based on the amount of reflected light (reproduction signal)from a pair of wobble pits 3. FIGS. 17B to 17D show reproduction signalsin the prepit region shown in FIG. 17A. A section Tc represents areproduction signal from the clock pit 2, a section Tw1 represents areproduction signal from the first wobble pit 3 a, and a section Tw2represents a reproduction signal from the second wobble pit 3 b.

[0007] The wobble pits 3 a and 3 b are shifted in opposite directions atthe same distance from the virtual track center 1. Therefore, in thecase where a beam spot for recording/reproducing passes along thevirtual track center 1, a decreased amount V1 of the reflected light inthe section Tw1 is equal to a decreased amount V2 of the reflected lightin the section Tw2, as shown in FIG. 17B. In the case where a beam spotis shifted to the first wobble pit 3 a side, the decreased amount V1 ofthe reflected light in the section Tw1 is increased, whereas thedecreased amount V2 of the reflected light in the section Tw2 isdecreased, as shown in FIG. 17C. On the other hand, in the case where abeam spot is shifted to the second wobble pit 3 b side, the decreasedamount V1 of the reflected light in the section Tw1 is decreased,whereas the decreased amount V2 of the reflected light in the sectionTw2 is increased, as shown in FIG. 17D.

[0008] As described above, when a beam spot is shifted from the virtualtrack center 1, a difference is caused between the decreased amounts V1and V2 of reflected light. According to the sample servo trackingsystem, by detecting the difference (tracking control signal) betweenthe decreased amounts V1 and V2 of reflected light, tracking control isconducted. According to the sample servo tracking system, all thereflected light from a disk is used, so that tracking control isunlikely to be influenced by a shift of a lens, a tilt of a disk, andthe like, which decreases a residual error in tracking control.

[0009] JP 4(1992)-301219 A discloses a method for increasing recordingdensity in the above-mentioned sample servo tracking system. Accordingto this method, wobble pits are shared by adjacent tracks. This methodcan double tracking density, compared with a conventional method.

[0010] Furthermore, in a conventional optical disk, the reproductionresolution of a signal is determined substantially by a wavelength λ ofthe reproduction light and a numerical aperture (NA) of an objectivelens, and a pit period of a detection limit is essentially λJ(2/NA).However, it is not easy to shorten a wavelength of reproduction light orincrease a numerical aperture of an objective lens. Therefore, variousattempts have been proposed for increasing recording density ofinformation by modifying a recording medium and a reproduction method.For example, JP 6(1994)-290496 A discloses a technique of enhancing areproduction resolution beyond a detection limit determined by awavelength of reproduction light and a numerical aperture of anobjective lens, using a DWDD method. According to the DWDD method,magnetic domain walls move successively by irradiation with a light beamfor reproduction, and the movement of the magnetic domain walls isdetected. According to this technique, when a reproduction layer that isa first magnetic layer, in which magnetic domain walls move upon beingirradiated with a light beam for reproduction, is separated magneticallybetween respective information tracks, a particularly satisfactoryreproduction signal is obtained.

[0011] As a method for magnetically cutting off a magnetic layer betweeninformation tracks, there is a method for conducting laser annealingbetween information tracks. However, it takes much time to conduct laserannealing. In order to solve this problem, a method for forming groovesand lands on an optical disk, and separating a magnetic domain wallmoving layer by the lands is proposed (see JP 11(1999)-120636 A).Furthermore, in an optical disk using both grooves and lands asrecording tracks, a method also is proposed for separating a magneticdomain wall moving layer by using a tilt of an inclined surface of thelands and grooves (see JP 11(1999)-120636 A).

[0012] However, in the case where information is recorded/reproducedwith respect to a high-density optical disk in accordance with theconventional sample servo tracking system, sufficient tracking accuracyis not ensured. This makes it difficult to realize a high-densityoptical disk. The problem regarding tracking accuracy becomesparticularly serious in the case of an optical disk conductingrecording/reproducing in accordance with the DWDD method described inthe prior art. This is because the DWDD method allowsrecording/reproducing to be conducted beyond the limit of a resolutionof an optical beam. In a conventional optical disk that does not adoptthe DWDD method, the optical resolution controls the recording density.Therefore, when a track pitch is narrowed, reproduction cannot beperformed due to crosstalk from an adjacent track. In order to avoidthis crosstalk, it is required to increase a track pitch to about 0.67times of λ/NA. However, according to the DWDD method,recording/reproducing can be performed even when a track pitch isnarrowed to about 0.49 times of λ/NA Therefore, tracking accuracy thatis much higher than that of a conventional optical disk is required togo along with the enhancement of track density by narrowing a trackpitch.

[0013] There are the following two problems for achieving such hightracking accuracy in the sample servo tracking system.

[0014] 1. Due to a tilt of an optical disk, a tracking error occurs,decreasing the tracking accuracy.

[0015] 2. An amplitude of a tracking control signal is varied between aninner periphery and an outer periphery of a disk, decreasing thetracking accuracy.

[0016] The first problem will be described. In tracking control inaccordance with the push-pull tracking system used in a conventionaloptical disk, a DC offset occurs in a tracking control signal by a tiltof a disk and a shift of a lens, and this error decreases trackingaccuracy. In contrast, according to the sample servo tracking system, aDC offset is not generated in a tracking control signal due to a tilt ofa disk, a shift of a lens, and the like. Therefore, it has beenconsidered in the prior art that the advantage of the sample servotracking system lies in that a tracking control signal is not fluctuateddue to a tilt of a disk, a shift of a lens, and the like. However, inthe sample servo tracking system, a tracking error that does not occurin a DC offset of a tracking control signal occurs due to a tilt of adisk, which decreases the tracking accuracy. This phenomenon becomesconspicuous in an optical disk that shares wobble pits between adjacenttracks, as disclosed in JP 4(1992)-301219A.

[0017]FIG. 18A shows a sample servo tracking system in the case wherewobble pits are not shared between adjacent tracks, and FIG. 18B shows asample servo tracking system in the case where wobble pits are sharedbetween adjacent tracks. In an information recording medium in FIG. 18B,the virtual track center 1 includes virtual track centers 1 a and 1 bhaving different polarities of reproduction signals from wobble pits.Compared with FIG. 18A, it is understood in FIG. 18B that the wobblepits of adjacent tracks are close to a light beam 5 to cause largeinterference. Thus, when track density is increased, the interference ofwobble pits between adjacent tracks is increased. As a result, atracking error occurs in the case where a tilt of a disk occurs, whichdecreases the tracking accuracy. Similarly, the interference of pitsbefore and after wobble pits also causes a tracking error.

[0018] As an example, FIG. 19 shows a reproduction signal in the casewhere an interval between the clock pit 2 and the first wobble pit 3 ais insufficient. In this case, the decrease in the amount V1′ ofreflected light in the section Tw1 is increased due to the influence ofthe clock pit 2. Thus, even in the case where a beam spot scans thevirtual track center 1, a tracking control signal (V1′-V2) does notbecome 0, and exact tracking control cannot be conducted.

[0019] Next, the second problem will be described in which an amplitudeof a tracking control signal is varied between an inner periphery and anouter periphery of a disk, and the tracking accuracy is decreased.

[0020]FIGS. 20A to 20C show how a reproduction signal is changed due tothe length of a prepit. In the case where a prepit is too short, anamplitude of a signal is too small to conduct tracking control, as shownin FIG. 20A As a prepit becomes longer, an amplitude of a signal isincreased as shown in FIG. 20B. When a prepit becomes longer comparedwith the state shown in FIG. 20B, an amplitude is decreased slightly toform a flat portion, as shown in FIG. 20C.

[0021] When an amplitude of a reproduction signal from the wobble pitsis changed depending upon the position on a disk, an amplitude of atracking control signal is varied depending upon the position on a disk,which decreases the reliability of tracking control. In order to avoidthis problem, a method is proposed for prescribing a wobble pit to belonger than a spot diameter to form a flat portion in a reproductionsignal, and using a signal of this portion to obtain a stable trackingcontrol signal (see JP 5(1993)-73929 A). However, if a wobble pit ismade longer, the prepit region becomes longer, so that linear density isdecreased.

[0022] Thus, in order to achieve high density in an optical diskadopting the sample servo tracking system, the size of a prepit in aprepit region and the interval between prepits are important.

[0023] In particular, according to the DWDD method, data is recordedonto a recording track cut off magnetically from an adjacent track, anda gradient of a heat distribution is used to conduct enlargementreproduction. Therefore, when tracking offset occurs during recordingand reproduction, reproduction characteristics are degraded remarkablyto cause an error. In order to conduct reproduction without an error, itis required to suppress a tracking error within ±0.04 μm. In an opticaldisk that is a medium to be replaced, it is very difficult. to suppressa tracking error (including those occurring due to convertibility andvibration between apparatuses) within ±0.04 μm.

[0024] The above-mentioned sample servo tracking system is excellent, inwhich a tracking error is unlikely to occur due to a tilt of a disk, ashift of a lens, and the like. However, it is very difficult to decreasea tracking error under various conditions even by using this system. Inthe sample servo tracking system, variations in an amplitude of atracking control signal in a disk and a detection error of a trackingposition occurring from a disk tilt become main factors of a trackingerror. Furthermore, the margin of a tracking error needs to coverfactors such as a control residual error involved in tracking control, atracking error occurring due to vibrations, and a detection error at atracking position.

[0025] In an ordinary optical disk, there are a control residual errorof about ±0.015 μm and a tracking error of ±0.02 μm caused byvibrations. In the DWDD method in which a tracking error margin is verysmall, i.e., ±0.04 μm, a detection error of only about ±0.005 μm of atracking position remains. Therefore, a detection error of a trackingcontrol signal in the sample servo tracking system, which has not been aproblem in the prior art, also becomes a serious problem. Since a trackpitch of a high-density optical disk is about 0.5 to 0.6 μm, a detectionerror of ±0.005 μm becomes about 1% of a track pitch. Therefore, inorder to realize a high-density optical disk, it is required to realizethe above-mentioned tracking position detection error in the smallestpossible servo region.

DISCLOSURE OF INVENTION

[0026] In view of the above, the object of the present invention is toprovide an information recording medium in which tracking control can beconducted with reliability and recording density can be increased.

[0027] In order to achieve the above-mentioned object, a firstinformation recording medium of the present invention, for reproducinginformation by irradiation with a laser beam condensed by an objectivelens with a numerical aperture NA, includes a disk-shaped substrate anda recording layer disposed on the substrate, wherein, on a surface ofthe substrate, a plurality of prepit regions and a plurality of dataregions are disposed alternately along spiral or concentric virtualtrack centers, each prepit region includes a pair of wobble pits fortracking servo, and a length L (μm) of the wobble pit along the virtualtrack center, a wavelength λ (μm) of the laser beam, and the NA satisfya relationship: 0.3≧L·NA/λ≧0.65. In the first information recordingmedium, irrespective of the position on the substrate, the amplitude ofa reproduction signal from a wobble pit becomes constant. Therefore, thefirst information recording medium allows tracking control to beconducted with reliability, and high-density recording to be conducted.Furthermore, the magnitude of interference caused by wobble pits inadjacent tracks shown in FIG. 18B is proportional to the length of thewobble pits. Therefore, by shortening the wobble pits, the interferencecan be reduced, and the tracking accuracy can be enhanced substantiallyin the case where an optical disk is tilted in a radius direction. Inthe present specification, “one track” refers to a single revolution.

[0028] In the first information recording medium, it is preferable thatflat portions are present on the surface of the substrate before andafter the wobble pits along the virtual track center, and a length M(μm) of the flat portion along the virtual track center and the spotdiameter D (μm) satisfy a relationship: 0.65≧(M/D). According to thisconfiguration, even when a tangential tilt occurs, tracking control canbe conducted with reliability. Therefore, for example, information canbe reproduced by using the DWDD method that is likely to be influencedby a residual error of tracking control, and the linear density can beenhanced.

[0029] A second information recording medium of the present invention,for reproducing information by irradiation with a laser beam condensedby an objective lens with a numerical aperture NA, includes adisk-shaped substrate and a recording layer disposed on the substrate,wherein, on a surface of the substrate, a plurality of prepit regionsand a plurality of data regions are disposed alternately along spiral orconcentric virtual track centers, each prepit region includes a pair ofwobble pits for tracking servo, flat portions are present on a surfaceof the substrate before and after the wobble pits along the virtualtrack center, and a length M (μm) of the flat portion along the virtualtrack center, a wavelength λ(μ) of the laser beam and the NA satisfy arelationship: 0.65≦M·NA/λ. In the second information recording medium,even when a tangential tilt occurs, tracking control can be conductedwith reliability. Therefore, for example, information can be reproducedby using the DWDD method that is likely to be influenced by a residualerror of tracking control, and a linear density can be enhanced.

[0030] In the first and second information recording media, one of thepair of wobble pits may be shared by two prepit regions that areadjacent in a radius direction of the substrate.

[0031] In the first and second information recording media, the prepitregion may be divided into a plurality of zones that are arrangedrepeatedly in accordance with a distance from a center of the substrate,the pair of wobble pits may be composed of a first wobble pit that hastwo possible arrangements and a second wobble pit that has two possiblearrangements, and the plurality of zones may have different combinationsof the arrangement of the first wobble pit and the arrangement of thesecond wobble pit. According to this configuration, by detecting theposition of a wobble pit, the radial movement direction of a laser beamspot can be detected.

[0032] In the first and second information recording media, grooves maybe formed in portions corresponding to the data regions in thesubstrate. According to this configuration, a tracking system can beadopted in which a tracking error of push-pull tracking using grooves iscorrected with wobble pits. In this case, compared with the trackingsystem using only wobble pits, the number of servo regions per track canbe decreased. Therefore, high-density recording can be conducted.Furthermore, a recording/reproducing system can be adopted in whichrecording tracks are cut off magnetically as in the DWDD method.Therefore, a linear density can be enhanced.

[0033] In the first and second information recording media, therecording layer may include a first magnetic layer, a second magneticlayer, and a third magnetic layer disposed in this order from anincident side of the laser beam, a Curie temperature of the firstmagnetic layer and a Curie temperature of the third magnetic layer maybe higher than a Curie temperature of the second magnetic layer, and therecording layer may be cut off magnetically between adjacent tracks.According to this configuration, information can be reproduced by theDWDD method.

[0034] In the first and second information recording media, the lengthsof the prepit regions along the virtual track center may be constant.

[0035] In the first and second information recording media, a distancebetween each center of the pair of wobble pits and an end of the prepitregion may be represented by an integral multiple of T/N, where T is alength of the prepit region along the virtual track center and N is aninteger of 5 or more. Herein, the end of a prepit region refers to theend of a prepit region on the side a laser beam spot enters duringreproduction. According to this configuration, it easily can bedetermined to which pattern a portion through which a light spot passesbelongs.

[0036] Furthermore, a first recording/reproducing method of the presentinvention is a method for recording/reproducing information byirradiating an information recording medium with a laser beam condensedby an objective lens with a numerical aperture NA, wherein theinformation recording medium includes a disk-shaped substrate and arecording layer disposed on the substrate, a plurality of prepit regionsand a plurality of data regions are disposed alternately along spiral orconcentric virtual track centers on a surface of the substrate, eachprepit region includes a pair of wobble pits for tracking servo, and alength L (μm) of the wobble pit along the virtual track center, awavelength λ (μm) of the laser beam, and the NA satisfy a relationship:0.3≦L·NA/λ≦0.65.

[0037] Furthermore, a second recording/reproducing method of the presentinvention is a method for recording/reproducing information byirradiating an information recording medium with a laser beam condensedby an objective lens with a numerical aperture NA, wherein theinformation recording medium includes a disk-shaped substrate and arecording layer disposed on the substrate, a plurality of prepit regionsand a plurality of data regions are disposed alternately along spiral orconcentric virtual track centers on a surface of the substrate, eachprepit region includes a pair of wobble pits for tracking servo, flatportions are present on the surface of the substrate before and afterthe wobble pits along the virtual track center, and a length M (μm) ofthe flat portion along the virtual track center, a wavelength λ (μm) ofthe laser beam, and the NA satisfy a relationship: 0.65≦M·NA/λ.

[0038] In another point of view, the present invention relates to arecording/reproducing system using an information recording medium and arecording/reproducing method of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

[0039]FIGS. 1A and 1B are plan views schematically showing an exemplaryconfiguration of an information recording medium of the presentinvention, and FIG. 1C is a cross-sectional view schematically showing areproduction state.

[0040]FIG. 2 is a plan view schematically showing a configuration of theinformation recording medium shown in FIGS. 1A, 1B, and 1C.

[0041]FIG. 3 schematically shows a configuration of segments of theinformation recording medium shown in FIG. 1A, 1B, and 1C.

[0042]FIG. 4 schematically shows a configuration of a prepit region ofthe information recording medium shown in FIG. 1A, 1B, and 1C.

[0043]FIG. 5 is a graph showing a relationship between a length of awobble pit and an amplitude of a tracking control signal.

[0044]FIG. 6 is a diagram showing an exemplary waveform of areproduction signal in the case where the length of a prepit is 0.32times or 0.63 times a spot diameter.

[0045]FIG. 7 is a graph showing a relationship between a length of aflat portion and a normalized tracking error.

[0046]FIG. 8 schematically shows a configuration of a prepit region ofthe information recording medium shown in FIGS. 1A, 1B, and 1C.

[0047]FIG. 9 is a view showing an exemplary reproduction signal of aprepit of the information recording medium shown in FIGS. 1A, 1B, and1C.

[0048]FIGS. 10A and 10B are plan views schematically showing anotherexemplary configuration of the information recording medium of thepresent invention, and FIG. 10C is a cross-sectional view schematicallyshowing a reproduction state.

[0049]FIG. 11 schematically shows a configuration of segments of theinformation recording medium shown in FIGS. 10A, 10B, and 10C.

[0050]FIG. 12 is a diagram showing positions of wobble pits and clockpits in the information recording medium shown in FIGS. 10A, 10B, and10C.

[0051]FIG. 13 is a cross-sectional view of the information recordingmedium shown in FIGS. 10A, 10B, and 10C.

[0052]FIGS. 14A to 14D schematically show the function of theinformation recording medium shown in FIGS. 10A, 10B, and 10C.

[0053]FIG. 15 schematically shows another exemplary configuration ofsegments in the information recording medium of the present invention.

[0054]FIG. 16 schematically shows an exemplary recording/reproducingapparatus used in a recording/reproducing method of the presentinvention.

[0055]FIGS. 17A to 17D schematically show a relationship between thearrangement of prepits and reproduction signals in a conventionalinformation recording medium.

[0056]FIG. 18A schematically shows an example of a relationship betweenprepits and a laser beam spot in the conventional information recordingmedium, and FIG. 18B schematically shows another example thereof.

[0057]FIG. 19 shows an exemplary reproduction signal in the conventionalinformation recording medium.

[0058]FIGS. 20A to 20C show other exemplary reproduction signals in theconventional information recording medium.

BEST MODE FOR CARRYING OUT THE INVENTION

[0059] Hereinafter, the present invention will be described by way ofillustrative embodiments with reference to the drawings.

[0060] Embodiment 1

[0061] In Embodiment 1, a first information recording medium of thepresent invention will be described. The information recording medium ofEmbodiment 1 is irradiated with a laser beam condensed by an objectivelens with a numerical aperture NA, whereby information is reproduced (orrecorded). The information recording medium is provided with adisk-shaped substrate. On the surface of the substrate, a plurality ofprepit regions and data regions are disposed alternately along spiral orconcentric virtual track centers. The prepit region includes a pair ofwobble pits for tracking servo. A length L (μm) of a wobble pit alongthe virtual track center and a spot diameter D (μm) of a laser beamalong the virtual track center satisfy a relationship: 0.3≦(L/D)≦0.65.Particularly, it is preferable that L and D satisfy a relationship:0.4≦(L/D)≦0.55. If the length of a wobble pit is within this range, evenif the length of a wobble pit is varied, variations in an amplitude of atracking control signal can be suppressed within several %. At thistime, even if the width of a wobble pit has variations in a range of 0.4times to 0.55 times a spot diameter, the amplitude of a tracking controlsignal hardly is influenced.

[0062] Herein, the virtual track center refers to a virtual line throughwhich a spot of a laser beam radiated for recording/reproducinginformation is to pass during recording/reproducing of information.Wobble pits for tracking control and data regions are formed along thevirtual track center.

[0063] Furthermore, a prepit refers to a convex portion or a concaveportion previously formed on a substrate for the purpose of generating aparticular signal during reproduction. A prepit includes, for example, aclock pit, a wobble pit, an address pit, and the like. The prepitgenerally has the shape of a circle, an oval, or a rectangle when viewedfrom above.

[0064] A spot diameter D (μm) (i.e., spot diameter in a tangentialdirection) of a laser beam along the virtual track center is representedby a formula: D =λ/NA, where NA represents a numerical aperture of anobjective lens of an optical head used for reproducing information and λrepresents a wavelength (μm) of a laser beam used for reproducinginformation. Thus, the information recording medium of Embodiment 1satisfies a relationship: 0.3≦L·NA/λ0.65, preferably 0.4≦L·NA/λ≦0.55.

[0065] In the information recording medium of Embodiment 1, thewavelength λ is, for example, in a range of 400 μm to 780 μm.Furthermore, the numerical aperture NA is, for example, in a range of0.55 to 0.85. Furthermore, the spot diameter D (=λ/NA) preferably is ina range of 0.47 μm to 1.42 μm.

[0066] In the information recording medium, prepit regions in whichprepits are formed are disposed separately on a disk. The arrangement ofthe prepit regions can be varied depending upon a rotation method of adisk. For example, the prepit regions may be disposed radially.Furthermore, the prepit regions may be disposed at a predeterminedinterval along the virtual track center. The lengths of the prepitregions along the virtual track center may be constant over the entireinformation recording medium, or may be longer toward an outerperiphery.

[0067] In the prepit region, prepits such as clock pits and address pitsare provided, if required, in addition to wobble pits. There are flatportions on the surface of the substrate before and after the wobblepits along the virtual track center. Herein, the flat portion refers toa region where prepits and pre-grooves (i.e., those previously formed onthe substrate) are not formed. More specifically, the flat portionrefers to a region where unevenness is not formed on the substrate. Itis preferable that a length M (μm) of the flat portion along the virtualtrack center and a spot diameter D (μm) of a laser beam along thevirtual track center satisfy a relationship: 0.65≦(M/D). Morespecifically, it is preferable that the length M (μm), the wavelength λ(μm), and the numerical aperture NA satisfy a relationship: 0.65≦M·NA/λ.

[0068] The information recording medium of Embodiment 1 includes arecording layer disposed on the substrate. In the data region,information is recorded onto the recording layer. As the recordinglayer, a layer made of a magnetic substance in which information isrecorded in magnetic domains can be used. As a method for recordinginformation, for example, an optical pulse magnetic field modulationsystem can be used that conducts recording by modulating a magneticfield while irradiating a laser beam in a pulse manner.

[0069] The information recording medium of Embodiment 1 allows trackingcontrol to be conducted with reliability and high-density recording tobe achieved, as described later in the examples.

[0070] Embodiment 2

[0071] In Embodiment 2, another information recording medium of thepresent invention will be described. The information recording medium ofEmbodiment 2 is irradiated with a laser beam condensed by an objectivelens with a numerical aperture NA, whereby information is reproduced (orrecorded). The information recording medium is provided with adisk-shaped substrate. On the surface of the substrate, a plurality ofprepit regions and data regions are disposed alternately along spiral orconcentric virtual track centers. The prepit region includes a pair ofwobble pits for tracking servo. There are flat portions on the surfaceof the substrate before and after the wobble pits along the virtualtrack center. A length M (μm) of the flat portion along the virtualtrack center and a spot diameter D (μm) of a laser beam along thevirtual track center satisfy a relationship: 0.65≦(M/D). Whenrepresented by using a wavelength λ (μm) of a laser beam to be radiatedand a numerical aperture NA of an objective lens, the length M, thewavelength λ, and the numerical aperture NA of an objective lens satisfya relationship: 0.65≦M·NA/λ.

[0072] The flat portion, the virtual track center, the prepit, the spotdiameter D (μm), the data region, and the recording layer are the sameas those in the information recording medium of Embodiment 1. Therefore,the description will not be repeated here.

[0073] The information recording medium of Embodiment 2 allows trackingcontrol to be conducted with reliability and high-density recording tobe achieved, as described later in the examples.

[0074] Embodiment 3

[0075] In Embodiment 3, a recording/reproducing method of the presentinvention will be described. According to a first recording/reproducingmethod of the present invention, the information recording medium ofEmbodiment 1 is irradiated with a laser beam condensed by an objectivelens with a numerical aperture NA, whereby recording/reproducing isconducted. According to a second recording/reproducing method of thepresent invention, the information recording medium of Embodiment 2 isirradiated with a laser beam condensed by an objective lens with anumerical aperture NA, whereby recording/reproducing is conducted.

[0076] As a recording/reproducing apparatus used in therecording/reproducing method of the present invention, a generalrecording/reproducing apparatus can be used.

[0077] According to the first recording/reproducing method of thepresent invention, a length L (μm) of a wobble pit along a virtual trackcenter, a wavelength λ (μm) of a laser beam, and a numerical aperture NAof an objective lens satisfy a relationship: 0.3≦L·NA/λ0.65, preferably0.4≦L·NA/λ≦0.55. According to the second recording/reproducing method ofthe present invention, a length M (μm) of the above-mentioned flatportion, a wavelength λ (μm) of a laser beam, and a numerical apertureNA of an objective lens satisfy a relationship: 0.65μM·NA/λ.

EXAMPLES

[0078] Hereinafter, the present invention will be described in detail byway of illustrative examples.

Example 1

[0079] In Example 1, an example of an information recording medium ofthe present invention will be described. FIG. 1A shows a plan view of aninformation recording medium 10 of Example 1. The information recordingmedium 10 is a magnetooptical disk of a sample servo tracking system.The information recording medium 10 has a disk shape with a diameter ofabout 50 mm, and is provided with a through-hole 100 at the center. Arecording region 101 is formed in a range of a radius of 11 mm to 25 mmon the information recording medium 10. A virtual track center 102 isdisposed in a spiral shape in the recording region 101. A track pitch(distance between the virtual track centers 102 adjacent in the radiusdirection) is 0.6 μm. FIG. 1B shows the arrangement of prepit regionsand data regions of the information recording medium 10 A plurality ofradially disposed prepit regions 110 and a plurality of radiallydisposed data regions 120 are placed alternately. In one track, 1280prepit regions 110 and 1280 data regions 120 are disposed.

[0080] As shown in FIG. 1C, the information recording medium 10 isirradiated with a laser beam 301 having a wavelength λ (μm) condensed byan objective lens 300 with a numerical aperture NA, wherebyrecording/reproducing is conducted.

[0081] As shown in FIG. 2, the recording region 101 of the informationrecording medium 10 is divided into a plurality of zones in the radiusdirection. The information recording medium 10 adopts a Zoned ConstantLinear Velocity (ZCLV) system in which recording/reproducing isconducted at a substantially constant linear velocity in the entirerecording region 101 by switching a rotation angular velocity on a zonebasis.

[0082]FIG. 3 is a partially enlarged view showing a configuration ofsegments 130 of the information recording medium 10. Referring to FIG.3, each segment 130 includes the prepit region 110 and the data region120. More specifically, the information recording medium 10 includes1280 segments per track. The data region 120 is used for recording userdata.

[0083]FIG. 4 schematically shows a configuration of the prepit region110. In the prepit region 110, prepits 111 are formed. The prepits 111include a clock pit 112, a pair of wobble pits 113, and an address pit114 in this order from the leading end (i.e., the side a spot of a laserbeam passes first during recording/reproducing of information). Thewidth (length in the radius direction) of each prepit 111 is 0.4 μm. Aspot LS of a laser beam moves in a direction represented by an arrow inFIG. 4. Furthermore, flat portions 11 p (see FIG. 8) are formed beforeand after the wobble pits 113 a and 113 b along the virtual track center102. In the flat portions lip, the prepits 111 are not formed.

[0084] The clock pit 112 is used for generating a synchronizing clocksignal, and disposed on the virtual track center 102.

[0085] A pair of wobble pits 113 include a wobble pit 113 a and a wobblepit 113 b. The wobble pits 113 a and 113 b are disposed at positionsshifted from the virtual track center 102 by a ½ track pitch. One ofwobble pits 113 is shared by the wobble pits of the adjacent tracks.Therefore, in the information recording medium 10, a track 103 a and atrack 103 b having different polarities of tracking control signals aredisposed alternately in the radius direction.

[0086] The address pit 114 is used for generating a signal regardingpositional information. One address pit 114 is provided per segment 130.Eighty address pits 114 form one address information. More specifically,16 (1280/80) pieces of address information are recorded per track in theinformation recording medium 10.

[0087] In the information recording medium 10, a length L (μm) of thewobble pit 113 along the virtual track center 102 and a spot diameter D(μm) of a laser beam to be radiated during reproduction of informationalong the virtual track center 102 satisfy a relationship: 0.3≦L/D≦0.65.In the case where the spot diameter D is about 1.1 μm, the length L is,for example, 0.4 μm.

[0088] For reference, FIG. 5 shows simulation results regarding changesin the amplitude of a tracking control signal when the length L of aprepit is changed. Herein, it is assumed that an optical system used forreproduction of information includes a semiconductor laser with awavelength λ of 0.65 μm and a lens with a numerical aperture NA of 0.6.Thus, the spot diameter D of a laser beam condensed onto the informationrecording medium 10 is about 1.1 μm (λ/NA). The horizontal axis in FIG.5 represents a length L (normalized by λ/NA) of a wobble pit along thevirtual track center 102. That is, a value on the horizontal axisrepresents an (L/D) value. For example, the value “1” on the horizontalaxis represents that the pit length L is 1.1 μm. Furthermore, thevertical axis in FIG. 5 represents an amplitude of a tracking controlsignal standardized with a reproduction signal on a mirror surface (flatportion without prepits).

[0089] It is understood from FIG. 5 that an amplitude of a trackingcontrol signal can be made substantially constant by prescribing thelength L (μm) of a wobble pit to be in a range of about 0.3 times toabout 0.65 times the spot diameter D (μm). More specifically, when thelength L (μm) and the spot diameter D (μm) satisfy a relationship:0.3≦(L/D)≦0.65, tracking control can be conducted with reliability.Furthermore, by prescribing the length L (μm) to be 0.65 times or lessthe spot diameter D (μm), high-density recording can be conducted.

[0090]FIG. 6 shows waveforms of reproduction signals when a length L′(μm) of a prepit along the virtual track center is 0.32 times (solidline in FIG. 6) or 0.63 times (broken line in FIG. 6) the spot diameterD (μm) along the virtual track center. A peak Pc represents areproduction signal of the clock pit 112, a peak Pwa represents areproduction signal of the wobble pit 113 a, and a peak Pwb represents areproduction signal of the wobble pit 113 b. In both the case of L′=0.32D and the case of L′=0.63 D, a flat portion is not present in areproduction waveform of a prepit. Thus, in this case, the centralposition of a prepit easily can be detected by using a differentialsignal of a reproduction signal or a tangential push-pull signal. Anoutput of PLL can be synchronized with the center of the clock pit 112based on information at the central position of a prepit. Furthermore,as described in Example 2, by detecting the positions of the wobble pits113 a and 113 b, the moving direction of a laser beam can be detected.

[0091] Next, the flat portions 11 p (see FIG. 8) formed before and afterthe wobble pits along the virtual track center 102 will be described. Inthe information recording medium 10, a length M (μm) of the flat portionalong the virtual track center 102 and a spot diameter D (μm) of a laserbeam along the virtual track center 102 satisfy a relationship:0.65≦(M/D). In the case where the spot diameter is about 1.1 μm, thelength M is, for example, 0.88 μm.

[0092] For reference, FIG. 7 shows simulation results regarding arelationship between the length M and the normalized tracking error.Herein, it is assumed that an optical system used for reproduction ofinformation is the same as that in the simulation in FIG. 5. Anormalized tracking error (%) on the vertical axis represents a valuecalculated by (tracking error/track pitch)×100. Furthermore, thehorizontal axis represents a value normalized by (λ/NA), i.e., the spotdiameter D (μm). More specifically, a value on the horizontal axisrepresents an (M/D) value. For example, the state where the value on thehorizontal axis is 1 represents that the length M of the flat portion is1.1 μm. Furthermore, in FIG. 7, a broken line represents the results inthe case where there is no tangential tilt, and a solid line representsthe results in the case where a tangential tilt occurs at 5 mrad.

[0093] In order to reproduce information with reliability, it isrequired to decrease a normalized tracking error even in the case wherea tangential tilt occurs. Particularly, in the case of using a recordinglayer for conducting reproduction by Domain Wall Displacement DetectionMethod (hereinafter, which may be referred to as a “DWDD method”),recording/reproducing characteristics during a tracking error are notsatisfactory, so that it is required to further decrease a normalizedtracking error. More specifically, in the case where a tangential tiltof 5 mrad occurs, it is preferable that a normalized tracking error is1% or less of a track pitch. Thus, it is understood from FIG. 7 that itis preferable to prescribe the length M of a flat portion to be 0.65times or more the spot diameter D. More specifically, when the length M(μm) and the spot diameter D (μm) satisfy a relationship: 0.65≦(M/D),tracking control can be conducted reliably.

[0094] Next, a configuration of the information recording medium 10 willbe described. FIG. 8 shows a partial cross-sectional view of theinformation recording medium 10 along the virtual track center 102.

[0095] Referring to FIG. 8, the information recording medium 10 includesa disk-shaped substrate 11 (hatching is omitted), and a dielectric layer12, a magnetic layer 13, a dielectric layer 14, and an overcoat layer 15stacked in this order on the substrate 11. In the prepit region 110,concave portions to be the clock pit 112, the wobble pits 113, and theaddress pits 114 are formed. Portions where prepits are not formed alongthe virtual track center 102 correspond to the flat portions 11 p. Alaser beam for recording/reproducing is radiated from the substrate 11side.

[0096] The substrate 11 can be made of, for example, polycarbonateresin, polyolefin resin, or the like. The dielectric layers 12 and 14can be made of, for example, a nitride such as SiN and AlN, an oxidesuch as SiO₂ and Al₂O₃, or a chalcogen type material such as ZnS andZnTe. The magnetic layer 13 can be made of, for example, a multi-layeredfilm composed of a combination of TbFeCo and TbFeCoCr, or TbDyFeCo,TbFe, and GdFeCo. UV-curable resin such as epoxy type UV-curable resinand urethane type UV-curable resin are examples that can be used for theovercoat layer 15.

[0097] Next, an exemplary method for producing the information recordingmedium 10 will be described with reference to FIG. 8. First, thesubstrate 11 is formed by injection forming using a stamper, a 2P method(photopolymer method), or the like. Then, the dielectric layer 12, themagnetic layer 13, and the dielectric layer 14 are formed successively.These layers can be formed by sputtering or vapor deposition.Thereafter, the overcoat layer 15 is formed. The overcoat layer 15 canbe formed by coating the dielectric layer 14 with UV-curable resin byspin coating and curing the UV-curable resin by irradiation withUV-light.

[0098] Next, a method for producing a master stamper for forming thepits on the substrate will be described. The master is formed by coatinga glass substrate with a photoresist, exposing a part of the photoresistto light by irradiation with a laser beam (cutting), and removing theirradiated portion. More specifically, the master is rotated at aconstant rotation number, and irradiated with a laser beam while thelaser beam is moved from an inner side to an outer side, whereby cuttingis conducted.

[0099] Herein, in the case where unevenness corresponding to the prepits111 is formed by cutting with a laser beam driven at a current modulatedwith the same pulse width irrespective of the position of the master,the length of a prepit becomes larger in accordance with the distancefrom the center of the master for the following reason. The rotationangular velocity of the master is constant, so that the relativemovement speed of the master with respect to a laser beam is increasedwith distance from the center of the master.

[0100] Herein, it is assumed that a laser beam (wavelength: 0.35 μm)driven at a constant driving pulse width is condensed by a lens with anumerical aperture NA of 0.9 to conduct cutting. It also is assumed thatthe spot diameter of a laser beam for reproducing a signal is 1.1 μm. Italso is assumed that the length of a prepit in the innermost track(radius: 11 mm) is equal to a spot diameter of a laser beam used forcutting. In this case, the length of a prepit in the innermost track(radius: 11 mm) becomes 0.35/0.9 =0.4 (μm)>0.3×(spot diameter of a laserbeam for reproducing a signal). On the other hand, the length of aprepit in the outermost track (radius: 25 mm) becomes (0.4/11)×25 =0.9(μm)>0.65 ×(spot diameter of a laser beam for reproducing a signal).Thus, when the rotation angular velocity of the master and the drivingpulse width of a laser beam are prescribed to be constant, it becomesdifficult to achieve the relationship between the length L (μm) of aprepit and the spot diameter D (μm) of a laser beam for reproducing asignal determined in the present invention, i.e., 0.3≦(L/M)≦0.65.

[0101] When the master of the information recording medium 10 isproduced, the driving pulse width of a laser beam used for cutting ischanged on the basis of 1000 tracks. More specifically, a disk ispartitioned into sections on the basis of 1000 tracks, and the length ofa prepit in the innermost track in the section is prescribed to be 0.4μm. In the same section, cutting is conducted at the same driving pulsewidth. A driving pulse width of a laser beam is set to be shortersuccessively in sections on the outer side, whereby cutting isconducted. Because of this, in each section, the length of a prepit inthe innermost track always becomes 0.4 μm. In each section, the lengthof a prepit of the outermost track becomes slightly longer than 0.4 μm.However, the width of each section is only 600 μm (1000 tracks), so thatthe length of a prepit can be set to be 0.43 μm or less. FIG. 9 showsexamples of a reproduction signal of a prepit with a length of 0.4 μm,and a reproduction signal of a prepit with a length of 0.43 μm. Thereproduction signal (represented by a broken line in FIG. 9) of a prepitwith a length of 0.43 μm has a decrease peak of the amount of reflectedlight wider than that of a reproduction signal (represented by a solidline in FIG. 9) with a length of 0.4 μm. However, with either prepit,the reproduction signal takes a local minimum value at the centralportion of a prepit, and a slope of the signal is changed largely beforeand after the central portion. Therefore, in the information recordingmedium of Example 1, the central position of a prepit easily can bedetected.

[0102] A nickel film is formed on the surface of the master thusproduced to obtain a stamper. The substrate 11 can be formed by a 2Pmethod or injection forming, using the stamper.

Example 2

[0103] In Example 2, another example of the information recording mediumof the present invention will be described. In an information recordingmedium 20 of Example 2, grooves (pre-grooves) corresponding to dataregions of a substrate are formed. In the information recording medium20, recording information is reproduced by the DWDD method. Furthermore,in the information recording medium 20, wobble pits are patterned on thebasis of 20 tracks, and a moving direction of an optical head can bedetected by detecting the wobble pits.

[0104]FIG. 10A is a plan view of the information recording medium 20 ofExample 2. The information recording medium 20 is a magnetooptical diskof a sample servo tracking system. The information recording medium 20has a disk shape with a diameter of about 50 mm, and is provided with athrough-hole 200 at the center. A recording region 201 is formed in arange of a radius of 11 mm to 25 mm on the information recording medium20. A virtual track center 202 is disposed in a spiral shape in therecording region 201. A track pitch (distance between the virtual trackcenters 202 adjacent in the radius direction) is 0.54 μm. FIG. 10B showsarrangement of prepit regions and data regions of the informationrecording medium 20. A plurality of radially disposed prepit regions 210and a plurality of radially disposed data regions 220 are placedalternately. In one track, 1280 prepit regions 210 and 1280 data regions220 are provided.

[0105] As shown in FIG. 10B, the information recording medium 20 ofExample 2 is characterized in that the lengths of the prepit regions 210are the same in a disk, compared with the information recording mediumof Example 1. In Example 1, the prepit regions 210 become wider towardthe outer peripheral side of the disk. In contrast, in the informationrecording medium 20, the prepit regions 210 have the same length, and inall the regions of the disk, the length of wobble pits and the intervalbetween wobble pits are the same in the prepit region 210. Therefore, inthe information recording medium 20, a more uniform tracking controlsignal can be generated in the disk, and tracking control with higherprecision can be conducted, compared with the information recordingmedium 10. Furthermore, the substrate of the information recordingmedium 10 is a flat plate, whereas pre-grooves are formed in the dataregion 220 in the information recording medium 20.

[0106] As shown in FIG. 10C, the information recording medium 20 isirradiated with a laser beam 301 with a wavelength λ (μm) condensed byan objective lens 300 with a numerical aperture NA, wherebyrecording/reproducing is conducted.

[0107] In the same way as in the information recording medium 10, therecording region 201 of the information recording medium 20 is dividedinto a plurality of zones in the radius direction (see FIG. 10). Theinformation recording medium 20 adopts a ZCLV system in whichrecording/reproducing is conducted at a substantially constant linearvelocity in the entire recording region 201 by switching a rotationangular velocity on a zone basis.

[0108]FIG. 11 is a partially enlarged view showing a configuration ofsegments 230 of the information recording medium 20. Referring to FIG.11, each segment 230 includes the prepit region 210 and the data region220. More specifically, the information recording medium 20 includes1280 segments per track. The data region 220 is used for recording userdata.

[0109] In the prepit region 210, prepits 211 are formed. The prepits 211include a pair of wobble pits 213 and an address pit 214 in this orderfrom the leading end (i.e., the side a spot of a laser beam passes firstduring recording/reproducing of information). The width (length in theradius direction) of each prepit 211 is 0.4 μm. Furthermore, flatportions are formed before and after the prepits 211 along the virtualtrack center 202.

[0110] The address pit 214 is a prepit similar to the address pit 114.described in Example 1.

[0111] In the information recording medium 20, a length L (μm) of thewobble pit 213 along the virtual track center and a spot diameter D (μm)of a laser beam radiated during reproduction of information satisfy arelationship: 0.3≦(L/D)≦0.65. In the case where the spot diameter D isabout 1.1 μm, the length L is, for example, 0.4 μm.

[0112] Furthermore, in the information recording medium 20, a length M(μm) of the flat portion along the virtual track center 202 and the spotdiameter D (μm) of a laser beam along the virtual track center 202satisfy a relationship: 0.65≦(M/D). In the case where the spot diameteris about 1.1 μm, the length M is, for example, 0.88 μm.

[0113] In the information recording medium 20, a clock pit is not formedin the prepit region 210. A synchronizing clock signal is generated bydetecting an edge of the pre-grooves 221 formed in the data region 220.

[0114] A pair of wobble pits 213 include a wobble pit 213 a(hereinafter, which may be referred to as a “first wobble pit 213 a”)and a wobble pit 213 b (hereinafter, which may be referred to as a“second wobble pit 213 b”) in this order from the leading end. Thewobble pits 213 a and 213 b are disposed at positions shifted from thevirtual track center 102 by a ½ track pitch. One of a pair of wobblepits 213 is shared by the prepit regions 210 that are adjacent in theradius direction. Therefore, in the information recording medium 20, atrack 203 a and a track 203 b having different polarities of trackingcontrol signals are disposed alternately in the radius direction.

[0115] In the information recording medium 20, the wobble pits 213 arepatterned. More specifically, the first wobble pit 213 a is disposed ata first position P1 a relatively close to the pre-groove 221 or at asecond position relatively distant from the pre-groove 221. Furthermore,the second wobble pit 213 b is disposed at a first position P2 arelatively close to the pre-groove 221 or at a second position P2 brelatively distant from the pre-groove 221. Thus, the wobble pits 213have four arrangement patterns: A pattern (P1 a, P2 b), B pattern (P1 b,P2 b), C pattern (P1 b, P2 a), and D pattern (P1 a, P2 a), dependingupon the arrangement of the first wobble pit 213 a and the second wobblepit 213 b. These patterns can be identified by generating timing signalscorresponding to P1 a, P1 b, P2 a, and P2 b from the synchronizing clocksignal, and sampling reproduction signals using the timing signals.

[0116]FIG. 12 schematically shows a relationship between the wobble pits213 and the address pits 214, and the length of the prepit region 210.As shown in FIG. 12, in each pattern (A pattern, B pattern, C pattern,and D pattern), each wobble pit 213, and each address pit 214 aredisposed so that each central position in a track direction ispositioned on a partition obtained by partitioning the prepit region 210into a plurality of regions with the same length. More specifically, thewobble pit 213 and the address pit 214 are disposed so that the distancebetween the center of a pit and the end of the prepit region 210 becomesan integral multiple of T/N (T is a length of the prepit region 210, andN is an integer of 5 or more). In an example shown in FIG. 12, thedistance between the center of the wobble pit 213 and the address pit214, and the end of the prepit region 210 is an integral multiple ofT/14. More specifically, the distance between the center of the firstwobble pit 213 a and the end of the prepit region 210 is 3T/14 or 4T/14,and the distance between the center of the second wobble pit 213 b andthe end of the prepit region 210 is 7T/14 or 8T/14. Furthermore, thedistance between the center of the address pit 214 and the end of theprepit region 210 is 11T/14. The arrangement shown in FIG. 12 is shownfor illustrative purpose, and the present invention is not limitedthereto.

[0117] In the case where the wobble pits 213 are arranged as shown inFIG. 12, and the length L of the wobble pit 213 satisfies 0.3≦L/D≦0.65,a signal amplitude is convex downwardly, and has a minimum at the centerof the wobble pit. Therefore, by comparing the position obtained bydividing the length of the prepit region 210 by an integer N (14 inExample 2) with a minimum position of a signal, it easily can bedetermined to which pattern a portion through which a light spot passesbelongs.

[0118] In the information recording medium 20, A pattern, B pattern, Cpattern, and D pattern are repeatedly arranged on the basis of 20 tracksfrom the inner peripheral side in the recording region 201. Morespecifically, A pattern (20 tracks), B pattern (20 tracks), C pattern(20 tracks), D pattern (20 tracks), A pattern (20 tracks), B pattern (20tracks) . . . are repeated in this order from the inner peripheral side.

[0119] Thus, in the information recording medium 20, a pair of wobblepits 213 consist of a first wobble pit 213 a having two types ofarrangement, and a second wobble pit 213 b having two types ofarrangement. Furthermore, the prepit region 210 is divided into aplurality of zones arranged repeatedly in accordance with a distancefrom the center of the substrate 21. A plurality of zones have differentcombinations of the arrangement of the first wobble pits 213 a and thearrangement of the second wobble pits 213 b. In the informationrecording medium 20, because of such arrangement, the moving directionof a beam spot easily can be detected during recording/reproducing ofinformation. For example, if it is found that a beam spot moves from Apattern to B pattern by detecting a signal from the wobble pit 213, itcan be determined that the beam spot has moved from an inner peripheralside to an outer peripheral side.

[0120] Next, the data region 220 will be described. In the data region220, a pre-groove 221 is formed along the virtual track center 202. FIG.13 is a partially enlarged cross-sectional view of the data region 220in the radius direction.

[0121] Referring to FIG. 13, the information recording medium 20includes a substrate 21, and a dielectric layer 22, a reproduction layer(first magnetic layer) 23, an intermediate layer (second magnetic layer)24, a magnetic recording layer (third magnetic layer) 25, a dielectriclayer 26, and an overcoat layer 27 stacked on the substrate 21 in thisorder. The reproduction layer 23, the intermediate layer 24, and themagnetic recording layer 25 form the recording layer 30 of theinformation recording medium 20. More specifically, the recording layerof the information recording medium 20 includes a first magnetic layer,a second magnetic layer, and a third magnetic layer disposed in thisorder from a laser beam incident side. The Curie temperature of thefirst magnetic layer and that of the third magnetic layer are higherthan that of the second magnetic layer.

[0122] The substrate 21 is made of the same material as that of thesubstrate 11. In the data region 220, pre-grooves 221 are formed on eachvirtual track center 202. The pre-grooves 221 are formed as linearconcave portions on the substrate 21. Flat lands 222 are formed betweenadjacent pre-grooves 221.

[0123] The dielectric layer 22 may be made of SiN and have a thicknessof 80 nm. The reproduction layer 23 may be made of GdFeCoCr and have athickness of 30 nm. The intermediate layer 24 may be made of TdDyFe andhave a thickness of 10 nm. The magnetic recording layer 25 may be madeof TbFeCo and have a thickness of 50 nm. The dielectric layer 26 may bemade of SiN and have a thickness of 80 nm. The reproduction layer 23 hasa compensating composition temperature of 150° C. and a Curietemperature of 270° C. The intermediate layer 24 has a Curie temperatureof 150° C., and a rare-earth metal composition always becomespredominant at a Curie temperature or lower. The magnetic recordinglayer 25 has a compensating composition temperature of 80° C. and aCurie temperature of 290° C. These layers can be formed by sputtering(e.g., DC magnetron sputtering, reactive sputtering, etc.). These layerscan have a desired composition ratio by varying the sputteringconditions and targets.

[0124] The overcoat layer 27 may be made of epoxyacrylate resin and havea thickness of 6 μm. The overcoat layer 27 may be formed by coating thedielectric layer 26 with resin by spin coating, and irradiating theresin with UV-light to cure it.

[0125] In the information recording medium 20, adjacent tracks are cutoff magnetically in the recording layer 30 of the data region 220. Thisis because a film to adhere to a side surface of a groove is very thin,and a side surface of a groove and a bottom surface of a groove aredifferent from each other in magnetic characteristics. Thus, in theinformation recording medium 20, information can be reproduced by theDWDD method. Hereinafter, reproduction in accordance with the DWDDmethod will be described with reference to FIGS. 14A to 14D.

[0126]FIG. 14A schematically shows a state of the data region 220 underthe condition that a laser beam is not radiated. An information signalis recorded onto the magnetic recording layer 25 as magnetizationinformation. When a laser beam is not radiated, the magnetic recordinglayer 25, the intermediate layer 24, and the reproduction layer 23 areexchange-coupled, so that magnetization information of the magneticrecording layer 25 is transferred to the intermediate layer 24 and thereproduction layer 23.

[0127]FIG. 14B schematically shows a state of the data region 220 when alaser beam is radiated. A laser beam moves relatively in the directionrepresented by an arrow with respect to the information recording medium20. When a laser beam is radiated and the temperature of each layer isincreased, a portion 24 p (represented by a shaded area in the figure)is formed in a part of the intermediate layer 24, where the temperaturereaches a Cure temperature or higher. In the portion 24 p,exchange-coupling between the magnetic recording layer 25 and thereproduction layer 23 is shut off. At this time, a magnetic domain wallof the reproduction layer 23 moves due to the gradient of magneticdomain wall energy density dependent upon the temperature. Therefore, anenlarged magnetic domain 23 a is present in the reproduction layer 23positioned on the portion 24 p. The information in the magnetic domain25 a of the magnetic recording layer 25 is transferred to the magneticdomain 23 a via the magnetic domain 24 a positioned forward of theportion 24 p (a moving direction of a laser beam relative to thesubstrate 21 is assumed to be a forward direction, which is representedby an arrow in FIG. 14B).

[0128] When a laser beam moves forward from the state in FIG. 14B, thetemperature of the magnetic domain 24 a is increased to become a portion24 p, as shown in FIG. 14C. At this time, a magnetic domain wall of themagnetic domain 23 b on the magnetic domain 24 b positioned forward ofthe portion 24 p moves. As shown in FIG. 14D, an enlarged magneticdomain 23 b is formed. The information of the magnetic domain 25 b istransferred to the enlarged magnetic domain 23 b via the intermediatelayer 24.

[0129] As described above, according to the DWDD method, informationrecorded in a recording mark is enlarged and transferred to thereproduction layer. Thus, the DWDD method allows a recording marksmaller than a spot diameter of a laser beam to be reproduced. In theinformation recording medium 20, recording is conducted by optical pulsemagnetic field modulation recording, and reproduction is conducted bythe DWDD method, whereby information with a particularly high densitycan be recorded/reproduced.

[0130] The arrangement of patterns, and the number of tracks included inone pattern are not limited to the above, and another configuration maybe used.

[0131] In Example 2, the information recording medium has been describedin which the pre-grooves 221 are formed in each track in the data region220. Another configuration may be used. For example, the pre-grooves 221may be formed on every other tracks in the data region 220. Regardingsuch an information recording medium 20 a, FIG. 15 schematically shows aconfiguration of the prepit region 210 and the data region 220 a. Theinformation recording medium 20 a is different from the informationrecording medium 20 only in that the pre-grooves 221 are formed on everyother track in the data region 220 a. Therefore, the description willnot be repeated here.

[0132] As shown in FIG. 15, in the data region 220 a, the pre-grooves221 are formed on every other track. Lands 222 also are formed on everyother track. More specifically, data is recorded on the pre-grooves 221in the track 203 a, and data is recorded on the lands 222 in the track203 b. In the information recording medium 20 a, adjacent tracks are cutoff magnetically because of the difference in step between thepre-grooves 221 and the lands 222. Therefore, information can bereproduced in accordance with the DWDD method.

Example 3

[0133] In Example 3, an exemplary recording/reproducing apparatus usedin the recording/reproducing method of the present invention will bedescribed.

[0134]FIG. 16 schematically shows a configuration of arecording/reproducing apparatus 400 of Example 3. Therecording/reproducing apparatus 400 records/reproduces information withrespect to a recording medium 401 according to the present invention.

[0135] The recording/reproducing apparatus 400 may include a motor 402,an optical pickup 403, a differential and additional amplifier 404, afocus signal generating circuit 405, a PLL 406, a tracking controlsignal detection circuit 407, a control circuit 408, a laser drivingcircuit 409, a data decoder 410, a data encoder 411, a magnetic headdriving circuit 412, and a magnetic head 413.

[0136] The motor 402 is controlled by the control circuit 408 androtates the recording medium 401. The optical pickup 403 is controlledby the control circuit 408 and records/reproduces data with respect tothe recording medium 401. The optical pickup 403 outputs signalsregarding a P-polarized light component and an S-polarized lightcomponent to the differential and additional amplifier 404. The opticalpickup 403 includes a semiconductor laser emitting a laser beam with awavelength λ, and an objective lens with a numerical aperture NA. Thedifferential and additional amplifier 404 conducts addition anddifferentiation of two signals input from the optical pickup 403, andoutputs an addition signal (pit signal) and a differential signal (MOsignal). The focus signal generating circuit 405 generates a signal forfocus control. The PLL 406 extracts a clock pit from the addition signalinput from the differential and additional amplifier 404, and generatesa clock signal for recording/reproducing. The tracking control signaldetection circuit 407 detects the amplitudes of a pair of wobble pitsbased on the input addition signal and clock signal, and generates atracking control signal by calculating the difference between the twoamplitudes. The control circuit 408 controls focusing, tracking, and themotor 402. The laser driving circuit 409 controls a laser power duringrecording and reproduction, and modulates a pulse of a laser. The datadecoder 410 decodes reproduced data based on the input differentialsignal. The data encoder 411 encodes recorded data. The magnetic headdriving circuit 412 drives the magnetic head 413 for recording of data.The magnetic head 413 generates a magnetic field in accordance with datafor recording.

[0137] The recording/reproducing method of the present invention can beperformed by using the recording/reproducing apparatus 400.

[0138] The present invention has been described by way of illustrativeembodiments. However, the present invention is not limited to theabove-mentioned embodiments, and is applicable to another embodimentbased on the technical idea of the present invention.

[0139] In the above example, a magnetooptical disk adopting the sampleservo tracking system is exemplified. However, an information recordingmedium of another system different from the sample servo tracking systemmay be used. For example, the present invention can be used in a disk ofa tracking system provided with guide grooves and wobble pits, in whichtracking usually is conducted using the guide grooves in accordance withthe push-pull system, and a tracking control error is corrected using awobble pit reproduction signal.

[0140] In Example 2, the case has been described in which wobble pitshave two arrangements. However, the information recording medium of thepresent invention is not limited thereto, and wobble pits may have twoor more arrangements.

[0141] The invention may be embodied in other forms without departingfrom the spirit or essential characteristics thereof The embodimentsdisclosed in this application are to be considered in all respects asillustrative and not limiting. The scope of the invention is indicatedby the appended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

INDUSTRIAL APPLICABILITY

[0142] As described above, according to the present invention, aninformation recording medium can be obtained in which tracking controlcan be conducted with reliability and high-density recording ofinformation can be conducted.

[0143] Furthermore, according to the recording/reproducing method of thepresent invention, information can be recorded onto an informationrecording medium with high density, and the information recorded withhigh density can be reproduced from the information recording medium.

1. An information recording medium for reproducing information byirradiation with a laser beam condensed by an objective lens with anumerical aperture NA, comprising a disk-shaped substrate and arecording layer disposed on the substrate, wherein, on a surface of thesubstrate, a plurality of prepit regions and a plurality of data regionsare disposed alternately along spiral or concentric virtual trackcenters, each prepit region includes a pair of wobble pits for trackingservo, and a length L (μm) of the wobble pit along the virtual trackcenter, a wavelength λ (μm) of the laser beam, and the NA satisfy arelationship: 0.3≦L·NA/λ≦0.65.
 2. An information recording mediumaccording to claim 1, wherein flat portions are present on the surfaceof the substrate before and after the wobble pits along the virtualtrack center, and a length M (μm) of the flat portion along the virtualtrack center, the wavelength λ (μm), and the NA satisfy a relationship:0.65≦M·NA/λ.
 3. An information recording medium according to claim 1,wherein one of the pair of wobble pits is shared by two prepit regionsthat are adjacent in a radius direction of the substrate.
 4. Aninformation recording medium according to claim 1, wherein the prepitregion is divided into a plurality of zones that are arranged repeatedlyin accordance with a distance from a center of the substrate, the pairof wobble pits are composed of a first wobble pit that has two possiblearrangements and a second wobble pit that has two possible arrangements,and the plurality of zones have different combinations of thearrangement of the first wobble pit and the arrangement of the secondwobble pit.
 5. An information recording medium according to claim 1,wherein grooves are formed in portions corresponding to the data regionsin the substrate.
 6. An information recording medium according to claim5, wherein the recording layer includes a first magnetic layer, a secondmagnetic layer, and a third magnetic layer disposed in this order froman incident side of the laser beam, a Curie temperature of the firstmagnetic layer and a Curie temperature of the third magnetic layer arehigher than a Curie temperature of the second magnetic layer, and therecording layer is cut off magnetically between adjacent tracks.
 7. Aninformation recording medium according to claim 1, wherein lengths ofthe prepit regions along the virtual track center are constant.
 8. Aninformation recording medium according to claim 7, wherein a distancebetween each center of the pair of wobble pits and an end of the prepitregion is represented by an integral multiple of T/N, where T is alength of the prepit region along the virtual track center and N is aninteger of 5 or more.
 9. An information recording medium for reproducinginformation by irradiation with a laser beam condensed by an objectivelens with a numerical aperture NA, comprising a disk-shaped substrateand a recording layer disposed on the substrate, wherein, on a surfaceof the substrate, a plurality of prepit regions and a plurality of dataregions are disposed alternately along spiral or concentric virtualtrack centers, each prepit region includes a pair of wobble pits fortracking servo, flat portions are present on a surface of the substratebefore and after the wobble pits along the virtual track center, and alength M (μm) of the flat portion along the virtual track center, awavelength λ (μm) of the laser beam and the NA satisfy a relationship:0.65≦M·NA/λ.
 10. An information recording medium according to claim 9,wherein one of the pair of wobble pits is shared by two prepit regionsthat are adjacent in a radius direction of the substrate.
 11. Aninformation recording medium according to claim 9, wherein the prepitregion is divided into a plurality of zones that are arranged repeatedlyin accordance with a distance from a center of the substrate, the pairof wobble pits are composed of a first wobble pit that has two possiblearrangements and a second wobble pit that has two possible arrangements,and the plurality of zones have different combinations of thearrangement of the first wobble pit and the arrangement of the secondwobble pit.
 12. An information recording medium according to claim 9,wherein grooves are formed in portions corresponding to the data regionsin the substrate.
 13. An information recording medium according to claim12, wherein the recording layer includes a first magnetic layer, asecond magnetic layer, and a third magnetic layer disposed in this orderfrom an incident side of the laser beam, a Curie temperature of thefirst magnetic layer and a Curie temperature of the third magnetic layerare higher than a Curie temperature of the second magnetic layer, andthe recording layer is cut off magnetically between adjacent tracks. 14.An information recording medium according to claim 9, wherein lengths ofthe prepit regions along the virtual track center are constant.
 15. Aninformation recording medium according to claim 14, wherein a distancebetween each center of the pair of wobble pits and an end of the prepitregion is represented by an integral multiple of T/N, where T is alength of the prepit region along the virtual track center and N is aninteger of 5 or more.
 16. A recording/reproducing method forrecording/reproducing information by irradiating an informationrecording medium with a laser beam condensed by an objective lens with anumerical aperture NA, wherein the information recording medium includesa disk-shaped substrate and a recording layer disposed on the substrate,a plurality of prepit regions and a plurality of data regions aredisposed alternately along spiral or concentric virtual track centers ona surface of the substrate, each prepit region includes a pair of wobblepits for tracking servo, and a length L (μm) of the wobble pit along thevirtual track center, a wavelength λ (μm) of the laser beam, and the NAsatisfy a relationship: 0.3≦L·NA/λ≦0.65.
 17. A recording/reproducingmethod for recording/reproducing information by irradiating aninformation recording medium with a laser beam condensed by an objectivelens with a numerical aperture NA, wherein the information recordingmedium includes a disk-shaped substrate and a recording layer disposedon the substrate, a plurality of prepit regions and a plurality of dataregions are disposed alternately along spiral or concentric virtualtrack centers on a surface of the substrate, each prepit region includesa pair of wobble pits for tracking servo, flat portions are present onthe surface of the substrate before and after the wobble pits along thevirtual track center, and a length M (μm) of the flat portion along thevirtual track center, a wavelength λ (μm) of the laser beam, and the NAsatisfy a relationship: 0.65≦M·NA/λ.